It is known to derive digital rotational-rate and angular-position data from a function generator, a synchro or resolver, by the amplitude method. In this system a carrier oscillation is amplitude modulated with respect to sine and cosine and the thus formed sine and cosine signals are fed to the opposite stator windings of the function generator. This creates in the rotor winding of the generator a signal from which it is possible by amplitude analysis to derive the angular-position information.
A function generator like a resolver or synchro is a robust signal generator by means of which it is possible to obtain rotary-position as well as rotation-rate information. There are basically two procedures for doing this. In one system the Doppler effect is used by analyzing the phase and/or frequency of the signal. The Doppler-effect method consists in the addition of a reference frequency to the rotor frequency of the function generator. The frequency differential between the reference frequency and the signal of the generator is used to determine the rotation rate. The phase shift between the generator signal and the reference signal is used to derive the angular position of the rotor. A disadvantage of this phase-type determination of the angular position is that it is dependent on the temperature of the time-constant elements of the function generator so that the phase might be off somewhat. In addition this system is very sensitive to outside influences so that very short connection lines must be used.
In the other known system the angular-position information is derived from an amplitude analysis of the generator signal, through the use of interference. This system of the invention is described in U.S. Pat. No. 3,720,866 and German patent document No. 3,619,285. The amplitude-type evaluation of the generator signal is not weighted with the above-given disadvantages. With the known amplitude-type method it is only, however, possible to derive the angular-position information from the amplitude relationship of the sine- and cosine-modulated carrier-frequency signal.
According to the system of U.S. Pat. No. 3,720,866 the angular position of a synchro can be determined with sufficient resolution and accuracy by means of a digital processor. It is, however, only limitedly possible to derive information about the rotation rate. This can be done for example by means of the following example:
With an angular resolution of 14 bits (16,384 angular steps per revolution) and a maximum rotation rate is 3000 rev/min, the smallest measuring dead time is 0.1 msec. Thus the frequency or count of the angular steps per second is equal to: EQU f=(3000.multidot.16,384)/60 sec=819,200 Hz,
and the number of angular steps for each dead time is EQU z=f.multidot.T.sub.t =8192 kHz.multidot.0.1 msec=81.92.
As a result at maximum speed of the generator the angle changes in spite of the high angular resolution only by about 82 steps per idle time. There must be at least one angular step per dead time for fine rotation-rate resolution, giving the following smallest rate resolution: EQU n.sub.min =n.sub.max /z=3000 min.sup.-1 /81.92=36.6/min.
The attainable rotation-rate range of 82 is not attainable by present-day servo systems. In order to get to higher ranges, the dead time must be correspondingly lengthened. This is however only possible by limiting the dynamics of the drive regulation and the operation at low speeds. For a very fast-acting servo drive it is therefore impossible to increase the dead time. According to the above-cited German patent document a good analog rotation-rate signal is obtained with a resolver. If a digital signal is to be derived from this analog signal by means of an analog/digital converter, there will be offset and drift problems in the speed control range and accuracy will suffer.